faces are lined and stripey in the view port after extruding
To get straight to the point if you try to extrude an object that is completely selected, it creates a duplicate of the object. Basically the weird "glitch" you are seeing is blender's lighting is having trouble processing your model because there's two of them in almost the exact same place.

Note: Extruding an entire object isn't a bad thing in blender, it can be used add another layer to the model, if you scale what you just extruded.

I tried my best to recreate the problem you were having in the following pictures:


The first image is an extruded but fully selected model





The second is after I extruded the model





This picture is of the same extruded model from picture two except I scaled the object on the X and Z axis (You can scale objects on specific axis by pressing your S key to scale and then X, Y, or Z for it so scale on one of those specific axis.)





In picture four I'm trying to show a huge use for extruding an object. If you were to bring an object with only one set of faces ("only one layer thick") it would only show one side and the other would be invisible.




Hope this helps sorry for the barrage of information, but I remember being new and I just don't want you to hit my same pot holes.